Advantages of connecting electric lights in series as compared with having all parallel connections are well known. The reasons include less interconnection wiring required, and ability to use lower voltage lamps. The lower voltage lamps can have lower cost, higher efficiency and longer life than parallel-connected lamps normally rated at the higher voltage of the power supply.
One of the most common uses of series light strings, for example, is for decoration and display purposes, particularly during Christmas and other holidays, such as for the decoration of Christmas trees and the like. One popular light set type currently available on the U.S. market comprises one or more strings of fifty miniature lamps (mini-lights) each, with each lamp typically having an operating voltage rating of approximately 2.4 volts, and whose filaments are connected in an electrical series circuit arrangement. If sets of more than fifty lamps are desired, the common practice is to provide a plurality of fifty miniature lamp strings, with the lamps in each string connected in electrical series, and with the plurality of strings being connected in a parallel circuit arrangement. Thus the sum of the voltage drops across lamps in each fifty-lamp series string would generally be on the order of 120 volts supplied by a common AC outlet.
If low voltage Christmas mini-lights were connected in parallel, total string current would be significant and connection wire size could limit flexibility. A transformer may have to be used to lower the voltage, since the normal full line voltage could not appear across such mini-lights due to their low voltage ratings and the small spacing between electrodes. A set of 100 parallel-connected lights would also require the wiring to be capable of handling 20 amperes if used with 200 milliampere rated bulbs. Connecting multiple strings `end-to-end` as is now customarily done would require 100 amperes if five such strings of 200 milliampere rated bulbs were connected end-to-end.
Series light connections thus have advantages over parallel light connections, particularly for connecting larger strings of low voltage lamps to be operated by common household and business AC outlet power supplies. However, a recognized drawback to series light connections is that when a single lamp fails to illuminate for any reason, there is an open circuit and remaining lamps will fail to light. Locating and replacing defective lamps in series light strings can be very frustrating and time consuming, particularly if multiple lamps are out. In larger strings especially, many lamps may have to be checked before finding a lamp that has failed. The problem is even more compounded when lamps fail for reasons that are not readily apparent visually, such as when series lamps have faulty socket connections.
Accordingly, various proposals have been made aimed at providing different shunt circuits that continue the operation of a series light string in the event one or more lamps fail. One type of shunt circuit uses an element of NTC (Negative Temperature Coefficient) electrical resistance material connected in parallel across each series lamp. When a shunted lamp operates normally, resistance through the NTC element is high. If the lamp fails, an increased current through the NTC element results in the element having a lowered resistance, in turn restoring current to a normal operating level through the light string.
Examples of proposed NTC element shunts being disclosed are found in U.S. Pat. No. 2,484,596 (specifying that the resistance across the NTC element is "effectively an open circuit" for the condition of an operating lamp, and referring to U.S. Pat. No. 2,258,646 as disclosing "suitable material" for the NTC element. In U.S. Pat. No. 2,258,646, the disclosed materials are metal oxides having specific resistances of on the order of 10,000 ohms and higher); U.S. Pat. No. 1,950,028 (specifying compressed metallic powder and sodium silicate to form an "insulative ring or spacer," to be used in place of a "usual glass ring"); U.S. Pat. No. 1,941,984 (specifying a metallic powder and sodium silicate material for fabricating NTC shunt elements); and U.S. Pat. No. 1,641,564 (specifying a metallic powder and sodium silicate material through which the "current flow in the circuit is substantially zero" at the moment of a failing lamp).
With shunt circuits of the NTC type, maintenance is simple because faulty lamps are visible from being the only ones out. Another potential advantage is that if the current through a series light string can be maintained at normal operating levels after failure of a lamp, remaining lamps can continue operating at their same level of brightness, as opposed to operating more brightly which could cause them to prematurely fail.
However, the proposed higher resistance NTC shunt materials have a number of disadvantages, particularly in the context of present day low voltage light strings. For one thing, their response times are low for transitioning to lower, lamp-like resistance levels, especially when starting from particularly lower temperatures, such as outdoors on a cold winter day. For example, the response time for even a typical 3000-ohm NTC thermistor could be on the order of tens of seconds at such temperatures, for the device to transition from higher to lower order resistances upon failure of a lamp. A reason for this is the very low current level at which high resistance NTC devices would initially operate. Another reason is that such lower order resistances would typically not be achieved until toward the very end of the device's temperature transition cycle period.
A further issue is the steady state resistance though the shunt when a lamp has failed. Ideally this resistance should be the same or slightly more than that of a normally operating lamp, in order to maintain current at normal operating levels in a series light string after a lamp has failed. However, the higher resistance NTC devices will typically not transition to low enough resistances to be feasible for use with today's low voltage, 2.4-3.5 volt mini-light lamps in series light strings when operating within acceptable (e.g., UL approved) temperature limits. Still another uncovered problem area is electrical stability of the parts; 3000-ohm and higher NTC thermistors have been observed to frequently fail when used as shunts in 2.4-3.5 volt mini-light lamps in series light strings, due to power or temperature overload. An explanation for this is the higher temperatures required for sufficiently low resistance transitioning, power dissipation maximizing at higher temperatures for an extended period, and the lower current ratings generally for higher impedance NTC thermistor devices.
In spite of prior art disclosures of NTC devices used as shunts in series light strings, the apparent absence of such devices in the marketplace demonstrates a need for alternative commercially feasible low cost shunt circuits that can reliably continue designed operation of series light strings when one or more lamps have failed. Indeed, in the case of low voltage Christmas mini-lights in the U.S., a common commercial expedient is the use of internal bulb shunts that provide short circuit paths when lamps burn out and do not reliably function. Internal bulb shunts also do not function when a lamp has a poor or loose connection in its socket, or if it falls out of its socket; in that case every lamp in the sting goes out and the faulty lamps can be difficult to find. Important considerations for series light shunts are cost and ease of manufacturing of components, size and packaging of components, ability to meet design point specifications, such as for low voltage lamp applications, and also competent performance under anticipated conditions of operation, such as outdoor or cold weather uses. In the event a lamp fails, whether it is because the lamp burns out or has a bad connection with its socket or falls out of its socket, it is desired that the series light string continue operating with remaining lamps continuing to operate at normal or only slightly diminished levels of brightness, which is not the case with internal bulb shunts.
Separately, it is known to place NTC thermistor devices in series with incandescent lamps in order to prevent damage from current surges that can result when lamps are first turned on. An NTC (Negative Temperature Coefficient) thermistor is a device having impedance that varies significantly in inverse proportion to the temperature of the device. In operation, a thermistor's temperature increases due to "self-heating" as current is passed through the device. The thermistors temperature is generally due primarily or at least in part to the ambient operating temperature and current flowing through the thermistor. When placed in series with a lamp, an NTC thermistor would have high impedance reducing the voltage across the lamp when the lamp is turned on. As current flows, the thermistor's impedance generally drops to a lower level, gradually increasing the voltage across the lamp to a normal full operating voltage.